In recent years, as mobile devices, such as a portable computer, a mobile phone, and a camera, have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having a high energy density and operation potential, a long cycle lifespan, and a low self discharge rate, into which much research has been carried out and which is now commercialized and widely used.
The secondary batteries may be recharged after being discharged such that the secondary batteries can be continuously used. However, the secondary batteries have different performances based on charge and discharge states. For this reason, efforts have been made to improve a charging method of the secondary batteries such that the performance of the secondary batteries can be enhanced.
FIG. 1 is a view showing a relationship between a charge current and a charge voltage in a conventional constant current (CC)-constant voltage (CV) charging method (hereinafter, referred to a CC-CV charging method), which is generally used as the charging method of the secondary battery as described above. As shown in FIG. 1, the CC-CV charging method is a method of charging the secondary battery at the maximum current until voltage of the secondary battery reaches a predetermined battery voltage and charging the secondary battery while gradually decreasing the charging current after the voltage of the secondary battery reaches the predetermined battery voltage.
FIG. 2 is a view showing a relationship between a charge period per step shown in FIG. 1 and a charge time. At a first step, the battery is charged with a constant current until the voltage of the battery voltage is increased to a charge limit voltage. When the voltage of the battery voltage is increased to the charge limit voltage, at a second step, the battery is saturated and the battery is charged while the charge current is gradually decreased. At this time, the voltage of the battery voltage reaches the maximum battery voltage. Subsequently, auxiliary charge is carried out for the battery based on the voltage of the battery. For a lithium ion battery, overcharge using overvoltage is not possible and trickle charge may damage the battery. For this reason, auxiliary charge is carried out while no trickle charge is applied. (The trickle charge is a method of continuously supplying a charge current having a size approximate to that of a self discharge current to a storage cell such that the storage cell always remains charged in a case in which the storage cell is used as an emergency electrode.)
During a constant current charge period (step 1), the battery is charged with a low constant current having a rate of 0.5 C or less for about one hour. As a result, a topping charge period (step 2) after the constant current charge period is relatively long. In addition, time or current is sensed to control the completion of charge. In the conventional CC-CV charging method, therefore, a C-rate is directly proportional to time.
In the conventional charging method as described above, a charge rate is about 85% or less when the battery is charged for 100 minutes. In order to achieve a charge rate of at least 90%, therefore, it is necessary to charge the battery for 130 minutes or more. Consequently, much time is needed to charge the battery.
Furthermore, in the conventional charging method, the performance of the battery is deteriorated in a case in which a high rate charge current is supplied to the battery in order to improve a charge speed.
In recent years, therefore, much research has been carried out into a charging method that is capable of stably charging a battery within a shorter time while reducing a deterioration rate of the battery and, in addition, there is a high necessity for an improved charging method.